The present invention relates to a cylinder lock with a rotatable key plug having a profiled keyway, said keyway being configured to accommodate an elongated, substantially flat key blade having a longitudinal profile groove, wherein                said keyway has two opposite lateral sides, one of which is substantially flat with a lower part thereof lying substantially in the same plane as an upper art thereof, and a profile rib located at said substantially flat lateral side,        said profile rib having a downwardly projecting tongue portion configured to extend inside said profile groove upon inserting said key into said keyway of said key plug of the lock.        
Such a lock with a keyway having a profile rib is previously known, e.g. from U.S. Pat. No. 5,715,717 (Widén) or U.S. Pat. No. 5,640,865 (Widén). Such locks and keys have proven to be very useful in that they provide an improved security. The keyway profile is quite distinguished from conventional keyways, and it is rather difficult to copy the associated keys. Moreover, they permit a great variation of the cross-sectional profile, which is a great advantage.